1. Field of the Invention
The invention relates to a quarter-bridge circuit for high currents, comprising a plurality of semiconductor components that can be turned off via a gate in each case and one freewheeling diode each, located opposite a midpoint, so that a total current in the quarter-bridge circuit is subdivided into a plurality of parallel current paths.
2. Discussion of Background
The intensive research in the field of power electronics has produced a large number of diverse component types. Of particular interest in this regard are those new switches which permit low-power and fast triggering. To date, however, only those components that require a complicated snubber network (e.g. GTO) continue to be available to the user for turning off high currents.
A further development of the known, user-friendly, finely structured components toward higher switching capacities harbors a number of imponderables and cost factors:
1. Higher powers cause larger chip surfaces and correspondingly larger contact surfaces. The problems which arise in the pressure contacting (necessary for large-area components) of the finest structures are not easy to grasp.
2. The housing and clamping system are complicated and expensive for large components.
3. The installation of the required, low-inductive triggering in the pressure contact housing is problematic.
4. With rising current intensity in the circuit, there is a rise in the risk of undesired electromagnetic interference (EMI=electromagnetic interference).
5. Leakage inductances in the load circuit can effect large overvoltage peaks with the reverse recovery of the diodes.
6. A consequence of an enlargement of the semiconductor switches is a reduction in yield during production. This disadvantage can be eliminated only partially using the so-called wafer repair technology.
There is no lack of attempts to overcome these problems, but nevertheless users have long wished to be able to control high powers with a low outlay on circuitry.
It must not be forgotten in the entire problem of powerful semiconductor components, however, that the user must have available a module in which the desired switch element (semiconductor chip) is packed in as user-friendly a fashion as possible. To achieve this does not mean to make available a semiconductor switch for high currents. Rather, the power semiconductor should be developed with regard to an advantageous module construction. In so doing, it is necessary to keep in view the most important circuit arrangements in the corresponding power range. These include, for example, bridge circuits for inverters, chopper circuits and the like.
The report "Abschaltbare Elemente der Leistungselektronik und ihre Anwendungen" ("Turnoff elements in power electronics and their applications"), Friedrich Speth, ETG Symposium 4th/5th May 1988, ETG Technical Report 23, pages 305-314 discloses the use of transistor modules which comprise a freewheeling diode in addition to the turnoff semiconductor component. The purpose of this connection is to keep the inductances as low as possible.
In order to widen the power range it is, moreover, proposed to connect a plurality of identical transistor modules in parallel. It is advantageous in this regard, for example, to connect emitter, collector and base by means of one low-inductive conductor rail in each case.
With GTOs, the situation is completely different. They are not connected in parallel, because the danger exists that given the slightest operating delays in individual components the current in the others becomes too high. However, even in conventional threephase converters problems can occur with the overcurrent if, for example, the electronic overcurrent protective devices fail. For this case, the named publication proposes to provide in the supply line a fuse which, given the occurrence of a fault in one branch, is blown by purposive firing of the remaining branches. The great disadvantage of this measure consists in that the entire converter is immediately put out of action.
The publication "High Voltage GTO Inverter Using Reverse Conducting GTO-Thyristor" by H. Saotome and S. Konishic discloses an inverter circuit having GTOs connected in series. Saturating inductances are provided in the lead in order to reduce the loading of the GTOs.